Various types coatings and sintered materials are used in numerous applications, such as in gas turbine engines to increase efficiency and/or protect components from heat and wear. Types of materials include thermal barrier coatings, abrasive coatings, abradable seals, and hard facing; hereafter referred to as coatings.
Since excessive blade/case clearances and disc/vane clearances within turbine engines allow the escape of gases which decreases engine efficiency, an abradable seal can be applied to minimize the clearances between the rotating and the stationary components. Thermal barrier coatings can be utilized to provide protection against high temperatures, while abrasive coatings can be used to prevent detrimental rub interactions and hard facing can be used to reduce wear.
Some coatings are applied by plasma or flame spraying; introducing particles (usually powders) into a hot gas stream or flame (respectively) which causes the particles to splat onto the substrate surface where they adhere and build up as a coating. Application of particles (i.e. AB-1) or short wires (i.e. Feltmetal.TM.) onto a substrate; by pre-sintering or partial sintering and then brazing, can be used to produce abradable coatings comprised of bonded particles, wires, or powders and void spaces; while bond coats can be produced by plasma spraying or vapor deposition. Bond coats are usually used in plasma spray and vapor deposition applications; a bond coat being a layer of metallic composition applied to the substrate before the coating is applied. U.S. Pat. Nos. 3,542,530, 3,676,085, 3,754,903, 3,879,831, 3,928,026, and 4,704,332, (incorporated herein by reference) describe various coatings, while U.S. Pat. Nos. 3,413,136, 4,055,705, and 4,321,311 (incorporated herein by reference) describe application techniques.
A common characteristic of these types of coatings is that the coating strength (cohesive strength) is relatively low; plasma sprayed or partially sintered particles are not well bonded to each other and there is usually porosity present. The strength of the coating is less than that of the substrate.
During engine maintenance, these coatings must frequently be removed; a process difficult to reliably preform and which frequently results in substrate damage. Various techniques have been employed for the removal of coatings: machining, chemical stripping, machining followed by chemical stripping (see for example U.S. Pat. Nos. 4,339,282, and 4,425,185; incorporated herein by reference), and grit blasting. For example, machining followed by chemical stripping requires that the component be held stationary while a machining tool removes the majority of the coating. A chemical solution, usually either a very strong acid or base, is then applied to the coating surface to disintegrate the remaining coating material. This technique requires extreme precision; without proper hardware alignment during machining damage to the substrate material occurs, while the chemical solution used tends to attack the substrate material. This process is also time consuming and labor intensive. Additionally, the chemical step, can produce hazardous waste. The individual processes of chemical stripping and machining also have the above described problems.
Another commonly used method, abrasive or grit blasting, also often results in damaged or destroyed components. This process consists of projecting abrasive particles in a compressed air stream against the coating. Since this technique requires immediate termination upon substrate exposure to prevent damage, it requires skilled operators.
Liquid jets above 10,000 psi, to the best of our knowledge, have not been utilized in the removal of coatings. Relatively low pressure liquid jets, 2,000 to 3,000 psi, have been applied in areas such as: cleaning applications, nuclear contamination removal, concrete scarifying, and barnacle and hull fouling removal, but not in an inorganic coating removal process.
Accordingly, an objective of this invention is to provide a convenient, cost effective, environmentally safe technique of removing coatings.